5.2.6  Solutes in the transpiration stream

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Much information available on the rate of solute transport through cell walls comes from studies of dye movement. Dye molecules are rather large compared to inorganic solutes which make up xylem sap and therefore only give an approximate idea of diffusivity of nutrient ions through a cell wall. Movement of dyes from the finest veins to leaf surfaces 100µm away takes about 30min, suggesting that diffusion rather than mass flow is responsible for solute distribution to cells. Water turns over in a whole leaf each 10–20min, demonstrating that solute movement through the cell wall apoplasm is one or more orders of magnitude slower than water movement.

To what extent do the conclusions about solute move-ment reached from these experiments apply to the natural solutes? The most abundant cation of the transpiration stream is potassium (K+), and its behaviour in the vessel network has been worked out in some detail although diffusivities of such small inorganic ions through cell walls have not yet been measured. Potassium does not concentrate in the narrowest vessels as the dyes do, because the xylem parenchyma and bundle sheath cells have carrier systems in their cell membranes which transport K+ to the symplasm. The tendency for apoplasmic K+ to become concentrated by water loss is thus counterbalanced by absorption into surrounding cells. High K+ concentrations recorded in leaf vessels (100–200mM) probably reflect an abundance of K+ in living xylem vessels prior to maturation and release of cell contents (Canny 1995).